1. Field of the Invention
The invention relates the field of semiconductor testing equipment and, more specifically, to the field of probe cards for semiconductor test systems.
2. Prior Art
Presently available probe cards make use of conventional printed circuit techniques using long unshielded and unguarded traces. These probe cards suffer from use of insulating and structural materials without regard for leakage characteristics. In addition, use of conventional printed circuit edge connectors add stray leakage and capacitance. These probe cards are not intended to address stray leakage and capacitance problems encountered in testing systems of today's technology. The previous state of the art did not require low-leakage, low-capacitance probe cards because tester equipment could perform these sensitive tests on only a few device pins, which made it impractical to use probe cards. As the size of devices become smaller and smaller, lower current, voltages, and capacitances must be measured. Test system vendors have met this demand with their latest generation of products, such as Hewlett-Packard's HP4062. However, there has been no corresponding progress in probe card technology.
Probe cards presently available fall into two categories: General purpose and high-performance alternating current (AC). The general purpose probe cards use conventional printed circuit design and layout technology, and use insulating and structural features (e.g. epoxy ring) which have provided adequate performance for previous generations of semiconductor technology, both in the development and production environments. However, these methods of construction impose leakage currents on the order of 25 times greater than the capability of the available test systems, and stray capacitance on the order of 10 times that of the test systems. Most of this leakage and stray capacitance is the direct result of the probe card printed circuit, but some of it results from the necessary printed circuit board edge connector, with the epoxy probe ring adding more leakage.
The high-performance AC probe cards were developed for the testing of finished product dice at operating speed. They provide features such as coaxial wiring, ground planes, and microstrip blade probes. These features address the major problems of this type of testing, such as impedence matching, (addressed by coaxial lead wiring and microstrip construction), and noise (addressed by ground plane design).
In order to achieve uniformity of impedance, microstrip ceramic blade probes are used in the high-performance AC probe cards. Probe card vendors have designed ceramic blade probes which use a ground plane on one side of the ceramic blade and a microstrip trace on the opposite side for connecting the probe to the chip. In current practice, the ground plane side of the blade is soldered to the ground plane of the probe card. The ground plane of the probe card usually consists of the entire top surface of the probe card.
In addition, in the latest generation of testing equipment one method of achieving low-leakage has been the use of a driven guard. This driven guard is an additional "shield" which is achieved by driving the ground planes to the same potential as the measured signal conductor. Since there is no perfect insulating material, any insulator has some conductance. By providing a potential at the non-signal end of the insulator which is identical to the signal of interest, no leakage current will flow through this insulator. This method of using driven guards has not, to the Applicant's knowledge, been previously applied to probe card technology.